This invention is related to pipe anchoring devices used in wellheads. More specifically, the invention pertains to anchoring devices utilizing slips for engaging a pipe and transferring the load to the casing head of a wellhead assembly.
A slip as referred to in the discussion herein is a wedge-like member having a somewhat cylindrical inner portion or front face and a tapered outer portion of back face. In use a pipe sometimes referred to as casing hereinafter is positioned through the slip on the front face and the back face is mounted in the bowl of the casing head portion of a wellhead assembly. The use of slips is well-known in the art, particularly in that segment related to oilfield wellhead assembly. Generally, all slips function somewhat the same in that the front face grasps the pipe and the back face grasps and engages the wellhead assembly as the pipe moves downwardly to a point where the weight of the pipe is supported or suspended by the slips. A difficulty experienced with the use of slips for the suspension of well casing is that the weight of the casing, particularly in deep wells, tends to place such large forces on the slips that the casing is collapsed or substantially radially compressed to a reduced diameter in the area where it contacts the slips. Generally, slips are designed so the final radial pressure exterted by the slip on the pipe or casing is such that the casing is not reduced in diameter to an extent which would prevent full gauge tools from passing through the casing in the area of the slips. Also they are designed such that stresses in the casing in the area of the slips are below the yield point.
In the prior art of slips used for the suspension of casing in a wellhead assembly, numerous designs are known which are operable to suspend casing in a wellhead assembly. However, the designs have certain drawbacks which are overcome by the pipe anchor of this invention. One of these prior art designs incorporates successive steps on the slip back face to provide a slip with a long travel and short effective length which will spread out the loads on the slip.
This construction is satisfactory but it is extremely expensive because of the precise tolerances required to effect simultaneous operation of the slips so they will seat simultaneously. Furthermore, another problem associated with this device is that the short effective length of the slip will cause the pipe or casing to be easily collapsed. Another known prior art device is constructed to increase the coefficient or friction between the backs of the slips and the surface of the bowl in the casing head. This is done by rough machining the surface in the bowl. Although this device will work it has not proven to be desirable because it does not provide sufficient gripping to firmly hold the pipe and prevent it from moving. Other pipe hangers are known which utilize a casing head bowl that is not tapered. This is accomplished by providing a tapered insert as a part of the slip. While this device can be made operable it has an inherent disadvantage in that it requires a considerable amount of high tolerance machining in the casing head bowl and on the segments of the slips for it to operate dependably and the cost of such machining can be prohibitive. Some slip constructions are known which are constructed to gall or tear the interior surface of the casing head bowl as they are loaded by utilizing teeth, protuberances, or other galling members on the back face of the slip segments. All these devices can be made operable; however, they require fine control of the materials from which the casing head and the slips are constructed so the coefficient of friction can be closely controlled as well as the motion of the slips both inwardly and downwardly.